Both.

I imagine the point is that if it is a private supply, as the owner of the transformer/PV/etc., the user can turn up the wick if the voltage drop is getting excessive.

Peter Hartley said:

Why is this not defined in BS7671?

Basically, public LV supply is ESQCR ... and BS 7671 is not permitted to repeat legislation in its requirements or definitions - although there are notes etc. along the lines "for public supplies in accordance with ESQCR ..."

Other supplies are "not public LV supplies" by pure logic,simply that, so why does BS 7671 need to define that?

I think just a minor mis-understanding of "standards speak" and what standards can and cannot do.

Thank you. I think BS 7671 needs to be clear. I understand the response from Chris Pearson above and I understand your explanation regarding a standard cannot be seen to contradict legislation. 

My interpretation has always been that a private supply is "privately generated" as there is no distinction made between HV grid point of connection and LV grid point of connection. Thus one can be excused for thinking that the private transformer is just part of the installation, such as cables, switchgear etc. 

I incorrectly assumed that it is due to the difficulty of keeping the privatly generated supply voltage constant. Thus some tweaking of the words would be helpful or include the explanation in the text.

Thank you, that does make sense.

A private supply is defined by BS 7671 as a supply that comes from a non-public source of energy. Therefore, the second option in your question is the correct one. A private supply can be produced by different types of generators, such as PV, diesel/gas/nuclear, or batteries. The first option in your question is not a private supply, because it still depends on the public grid supply at the HV level. The private transformer only changes the voltage level, not the source of energy.

Peter Hartley said:

My interpretation has always been that a private supply is "privately generated" as there is no distinction made between HV grid point of connection and LV grid point of connection.

Well, there's a huge difference (legally) between a public LV supply, and one derived from a private transformer, where the public supply is provided at HV.

In general, supplies received at HV are outside the scope of BS 7671 (by definition).

"grid-independent" is another issue again.

AMK said:

A private supply is defined by BS 7671 as a supply that comes from a non-public source of energy.

BS 7671 certainly talks about private sources and private transformers - both not really directly relevant in terms of the particular issue we are discussing?

Where is "private supply" defined in the standard? (certainly not in Part 2).

Thanks, that's my thiughts as well. But do you know where this in the book this is defined? 

Quite simpy, if you receive a supply at HV, and under certain other conditions according to ESQCR if you take a supply at LV, you will have a connection agreement that tells you what you should expect at the supply terminals, which will not necessarily be U₀ + 10 % / - 6 % for a "standard" LV public supply.

There's no way that BS 7671 could know what terms you have agreed with the supplier ... and therefore the standard can't be specific about this.

Apologies for the spelling, trying to watch the rugby and type at the same time. Do you know where this distinction is made?